Re: [PATCH v4 net-next 01/10] mptcp: Fix up subflow's memcg when CONFIG_SOCK_CGROUP_DATA=n.

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Aug 14, 2025 at 08:08:33PM +0000, Kuniyuki Iwashima wrote:
> When sk_alloc() allocates a socket, mem_cgroup_sk_alloc() sets
> sk->sk_memcg based on the current task.
> 
> MPTCP subflow socket creation is triggered from userspace or
> an in-kernel worker.
> 
> In the latter case, sk->sk_memcg is not what we want.  So, we fix
> it up from the parent socket's sk->sk_memcg in mptcp_attach_cgroup().
> 
> Although the code is placed under #ifdef CONFIG_MEMCG, it is buried
> under #ifdef CONFIG_SOCK_CGROUP_DATA.
> 
> The two configs are orthogonal.  If CONFIG_MEMCG is enabled without
> CONFIG_SOCK_CGROUP_DATA, the subflow's memory usage is not charged
> correctly.
> 
> Let's wrap sock_create_kern() for subflow with set_active_memcg()
> using the parent sk->sk_memcg.
> 
> Fixes: 3764b0c5651e3 ("mptcp: attach subflow socket to parent cgroup")
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  mm/memcontrol.c     |  5 ++++-
>  net/mptcp/subflow.c | 11 +++--------
>  2 files changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8dd7fbed5a94..450862e7fd7a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5006,8 +5006,11 @@ void mem_cgroup_sk_alloc(struct sock *sk)
>  	if (!in_task())
>  		return;
>  
> +	memcg = current->active_memcg;
> +

Use active_memcg() instead of current->active_memcg and do before the
!in_task() check.

Basically something like following:

	memcg = active_memcg();
	/* Do not associate the sock with unrelated interrupted task's memcg. */
	if (!in_task() && !memcg)
		return;

>  	rcu_read_lock();
> -	memcg = mem_cgroup_from_task(current);
> +	if (likely(!memcg))
> +		memcg = mem_cgroup_from_task(current);
>  	if (mem_cgroup_is_root(memcg))
>  		goto out;
>  	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg1_tcpmem_active(memcg))
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 3f1b62a9fe88..a4809054ea6c 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -1717,14 +1717,6 @@ static void mptcp_attach_cgroup(struct sock *parent, struct sock *child)
>  	/* only the additional subflows created by kworkers have to be modified */
>  	if (cgroup_id(sock_cgroup_ptr(parent_skcd)) !=
>  	    cgroup_id(sock_cgroup_ptr(child_skcd))) {
> -#ifdef CONFIG_MEMCG
> -		struct mem_cgroup *memcg = parent->sk_memcg;
> -
> -		mem_cgroup_sk_free(child);
> -		if (memcg && css_tryget(&memcg->css))
> -			child->sk_memcg = memcg;
> -#endif /* CONFIG_MEMCG */
> -
>  		cgroup_sk_free(child_skcd);
>  		*child_skcd = *parent_skcd;
>  		cgroup_sk_clone(child_skcd);
> @@ -1757,6 +1749,7 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
>  {
>  	struct mptcp_subflow_context *subflow;
>  	struct net *net = sock_net(sk);
> +	struct mem_cgroup *memcg;
>  	struct socket *sf;
>  	int err;
>  
> @@ -1766,7 +1759,9 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
>  	if (unlikely(!sk->sk_socket))
>  		return -EINVAL;
>  
> +	memcg = set_active_memcg(sk->sk_memcg);
>  	err = sock_create_kern(net, family, SOCK_STREAM, IPPROTO_TCP, &sf);
> +	set_active_memcg(memcg);
>  	if (err)
>  		return err;
>  
> -- 
> 2.51.0.rc1.163.g2494970778-goog
>